Quantum gravity is one of the greatest puzzles of all time: to combine into one unified picture the two most important achievements of 20th century physicsâthe quantum theory, and Albert Einsteinâs theory of space, time and gravity. Each of these two remarkably powerful theories dramatically pushed back the boundaries of our understanding and forced us to think in completely new ways about the universe in which we live. Successfully combining them together promises to yield the deepest insights physicists have every achieved into how our universe works.

What did Einstein discover that made us think in new ways? That space and time are not separate entities as Galileo and Newton had conceived, but are intimately related aspects of a single, four-dimensional reality called spacetime. And that this spacetime is not merely a static backdrop on which everything else happens, but is in fact a dynamical player in the universeâs great dance: matter causes spacetime to warp, and this warping, in turn, affects the way matter moves, the net result being a beautiful, geometrical explanation of the phenomenon we call gravity. It superseded Newtonâs theory of gravity and predicted a host of new phenomena: black holes, gravity waves, gravitational lensing; and is now the very framework within which we understand our expanding universe.

What did the quantum theory reveal? That in the ultra-microscopic world of atoms and subatomic particles, nature operates in very bizarre ways: a single particle can behave as if it is in two places at once; a pair of particles, even a great distance apart, can behave in some ways as a single entity. This led to discoveries such as: antimatter (which when combined with matter will annihilate both, releasing pure energy) and vacuum polarization (whereby âemptyâ space is never really empty, but rather is seething with âvirtualâ particles continuously being created out of nothing and returning back to nothing), as well as a host of practical devices like the transistor and the laser, the basis of much of todayâs computing and communications technologies.

Why the desire to combine relativity theory (i.e. gravity) with quantum theory? First, such a unified theory will be needed to answer many profound questions, including: How did the Big Bang begin? Or: What is happening deep inside a black hole? But more importantly, the history of physics has taught us again and again that successful unifications of seemingly disparate theories are a natural direction of progress in physics, invariably leading to deeper insights intoâand more profound questions aboutâthe workings of our mysterious universe. The search for a quantum theory of gravity, which strongly challenges the entire foundation of our understanding of the universe, promises to be the greatest unification of all.

For a more detailed description of the history and current state of quantum gravity research, click here.

Suggested Reading

Julian Barbour, The End of Time: The Next Revolution in Physics (Oxford University Press, 2001)
Craig Callender and Nick Huggett (Eds.), Physics Meets Philosophy at the Planck Scale: Contemporary Theories in Quantum Gravity (Cambridge University Press, 2001)
Albert Einstein, Relativity: The Special and the General Theory (Crown Pub, 1995) and The Meaning of Relativity (Princeton University Press, 1966)
Brian Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (Vintage Books, 2000)
Stephen Hawking, A Brief History of Time (Bantam Doubleday Dell, 1998)
Roger Penrose, The Emperorâs New Mind: Concerning Computers, Minds, and the Laws of Physics (American Philological Association, 1989)
Lee Smolin, Three Roads to Quantum Gravity (Basic Books, 2002) and The Life of the Cosmos (Oxford University Press, 1997)
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